1. Field of the Invention
The present invention relates to a mold protection method for a mold clamping apparatus, which method detects an anomaly such as presence of a foreign object on the basis of a change in a physical quantity in a mold protection zone.
2. Description of the Related Art
A conventional toggle-type mold clamping apparatus for clamping a mold of an injection molding machine is disclosed in, for example, Japanese Patent Publication (kokoku) No. 6(1994)-61806. As disclosed in this publication, a toggle-type mold clamping apparatus includes a toggle mechanism which connects a movable platen for supporting a movable mold half and a crosshead advanced and retracted by a drive unit, and has a function of transmitting pressing force of the crosshead to the movable platen while amplifying the force. In such a mold clamping apparatus, when the toggle mechanism is completely extended, a predetermined mold clamping force determined on the basis of an extension of tie bars is generated. As shown in FIG. 10, in a mold clamping operation, high-speed mold closing is typically performed from a mold open position Xa, and the operation mode is switched to low speed, low pressure mold closing at a predetermined low speed, low pressure changeover position Xb. The period during which the low speed, low pressure mold closing is performed serves as a mold protection zone, during which a molded product not having been properly ejected or the like is detected as a foreign object. When a predetermined high-pressure changeover position Xc is reached, the operation mode is switched to high-pressure mold clamping so as to clamp the mold under high pressure. In FIG. 10, Xd shows a mold clamping end position. Load torque T of a drive motor for driving the mold clamping apparatus changes as shown in FIG. 10 during the mold clamping operation.
Incidentally, unlike a direct-pressure-application-type mold clamping apparatus, because of its operation principle, a toggle-type mold clamping apparatus has a drawback in that slight expansion or contraction of a mold and tie bars, stemming from disturbing factors such as heating temperature of the mold and ambient temperature, causes a considerable change in mold clamping force, which results in deterioration in quality, in particular at the time of molding of precision products. FIG. 11 shows a change in mold clamping force Fm with time for the case where the correct value (target value) of mold clamping force Fm is 400 kN. As is apparent from FIG. 11, during a period in which the mold temperature elevates, the mold clamping force Fm increases from 400 kN to 500 kN because of thermal expansion of the mold. After completion of the temperature elevation, since heat is transferred from the mold to the tie bars, the tie bars expand, whereby the mold clamping force Fm gradually decreases. Notably, thermal expansion of the mold is a factor which increases the mold clamping force Fm, and thermal expansion of the tie bars is a factor which decreases the mold clamping force Fm.
As described above, in a toggle-type mold clamping apparatus, disturbing factors such as heating temperature of a mold and ambient temperature are influential factors which must be taken into consideration so as to accurately maintain the mold clamping force Fm. Japanese Patent Application Laid-Open (kokai) No. 62(1987)-32020 discloses a mold clamping force control method which can cope with such disturbing factors. In the disclosed method, the thickness of a mold or a mold clamping force during a molding operation is detected by means of mold-thickness detection means consisting of an optical or magnetic scale supported on a stationary mold plate and a position detector disposed on a movable mold plate, and a correction value determined from the detected thickness and its target value is fed back to mold-thickness adjustment means, whereby mold clamping force is maintained constant.
Meanwhile, in the above-described toggle-type mold clamping apparatus, in general, a predetermined mold protection zone is set in a mold closing zone, and an anomaly such as presence of a foreign object is detected on the basis of a change in a physical quantity in the mold protection zone so as to protect a mold. In such a case the foreign object is mainly a molded product not having been ejected during mold opening and remaining within the mold.
Incidentally, in some cases, sheet-shaped products having a thickness of about 0.1 mm are molded. Due to its light weight, such a molded product is likely to adhere to the inner surface of the mold by means of static electricity. Accordingly, in order to detect a molded product of such a kind, the end position of the mold protection zone must be set to be located rearward, by at least 0.1 mm, of the closed point of the mold. In addition, in the case where the closed point of the mold is detected on the basis of change in a physical quantity, the end position of the mold protection zone must be set to be located rearward of the closed point in order to avoid interference between detection of a foreign object and detection of the closed point.
However, even in the case where the end position of the mold protection zone is accurately set so as to meet the above-described requirements, when the above-described thermal expansion of a mold and that of tie bars occur upon an increase in temperature of the mold, the closed point of the mold shifts forward or rearward, whereby detection of an anomaly such as presence of a foreign object becomes impossible or erroneous detection occurs, under a certain condition that a thin product is molded and the closed point of the mold is detected on the basis of change in a physical quantity.